Hi OP, i work with teenage girls.
my advice for the immediate is to regulate yourself. There is no immediate danger to her safety.
you and your daughter in this moment tonight are both dysregulated and panicking about a perceived future that may or may not happen. Your fear that she will be hurt by someone who exploits or manipulates her. She also fears that she will be hurt but by losing someone she thinks she loves and losing the connection, belonging or whatever she thinks she's getting out of it. Your fears may make more sense to us adults but her fear is huge to her and overwhelming her capacity for rational thought.
right now she is safe in her room. Distress, panic and calm are all contagious - choose contagious calm.
your daughter is correct that you don't understand her feelings, tell her that and that you want to understand.
these are two teenage girls, one who is mentally unwell and one who thinks her mum doesn't want understand.
if possible, get the parents and girls together to facilitate the relationship in the safest way possible while getting your daughter into counseling and getting some psycho education on manipulative or codependent relationships.
it is paramount that your daughter sees you as someone who she can go to and will try to understand when things are scary. With kindness, this extreme reaction is showing you can't provide a safe space for her to tell you her feelings.
it is normal and understandable to worry for her safety but you must also provide emotional safety in her relationship with you and for her to express her feelings no matter how scary they are.
She is probably dying for a grown up to take charge and put some boundaries around this but a panicked, non-listening grown up can't do that.